To clarify the mechanism leading to the development and rupture of intracranial aneurysms, tensile strength and viscoelastic parameters of 22 human saccular aneurysms were investigated. Meridional and circumferential strips from the thin and the thick part of the aneurysm sack and 18 control strips from the basilar artery of 8 patients with pathologies not affecting the cerebral arterial system were studied. The length of the strips was increased in 200-µm steps, while distending force was recorded. Tensile strength and viscoelastic parameters were computed. In both directions, tensile strength of thick strips was significantly lower than that of controls. In the meridional direction, tensile strength of thin strips was significantly larger than that of thick ones (14.5 ± 4.1 × 10<sup>6</sup> vs. 7.5 ± 2.0 × 10<sup>6</sup> dyn/cm<sup>2</sup>, p < 0.05). In the circumferential direction, thin strips tore at lower strain values than thick ones (29 ± 4 vs. 55 ± 16%, p < 0.05). Viscoelastic parameters changed in parallel. In circumferential direction, values of thick and thin strips were significantly lower than those of controls. In the meridional direction, values of thin strips were significantly higher than those of the thick ones. These observations show that characteristic mechanical deterioration and steric inhomogeneities accompany the loss of smooth muscle cells and the derangement of connective tissue elements in the wall of intracranial aneurysms, which may explain certain steps in their initiation, enlargement and rupture.